Super absorbent polymer (SAP) is a synthetic polymer material capable of absorbing moisture from about 500 to about 1,000 times its own weight, and it has been also called a super absorbency material (SAM), an absorbent gel material (AGM) and so on. Such super absorbent polymers started to be practically applied in sanitary products, and now they are widely used for preparation of various products, for example, hygiene products such as disposable diapers for children, water retaining soil products for gardening, water stop materials for the civil engineering and construction, sheets for raising seedling, fresh-keeping agents for food distribution fields, or the like.
The known preparation process for such super absorbent polymers includes a process by reverse phase suspension polymerization and a process by aqueous solution polymerization. Among them, the process by reverse phase suspension polymerization is disclosed in Japanese Laid-Open Patent Publication No. Sho 56-161408, Japanese Laid-Open Patent Publication No. Sho 57-158209, and Japanese Laid-Open Patent Publication No. Sho 57-198714, and the like. And, the process by aqueous solution polymerization includes a thermal polymerization method in which a hydrous gel phase polymer is polymerized while being broken and cooled in a kneader equipped with a plurality of shafts, and a photo-polymerization method in which an aqueous solution of a high concentration is irradiated by UV and the like on a belt to simultaneously progress polymerization and drying.
Meanwhile, absorption speed, which is one of the important properties of super absorbent polymer, is related to surface dryness of a product contacting skin such as a diaper. In general, the absorption speed may be improved by increasing the surface area of super absorbent polymer.
For example, a method of forming a porous structure on the surface of super absorbent polymer particles using a blowing agent is applied. However, with common blowing agents, a sufficient amount of a porous structure cannot be formed, and thus, increase rate of the absorption speed is not high.
For another example, a method of reassembling fine particles obtained in the preparation process of super absorbent polymer to form porous particles of irregular shapes, thus increasing the surface area is used. However, although the absorption speed of super absorbent polymer may be improved by this method, centrifuge retention capacity (CRC) and absorbency under pressure (AUP) of the polymer are relatively lowered. As such, since the properties of absorption speed, centrifuge retention capacity, absorbency under pressure, and the like of super absorbent polymer have trade-off relationship, a preparation method capable of simultaneously improving these properties is urgently needed.
Meanwhile, since the monomers used for the preparation of super absorbent polymer are strong acid, they have adverse effects on the human body such as skin irritation with a small amount, and thus, it is important to reduce the content of non-reacted monomers in the super absorbent polymer.
Since gel type super absorbent polymer is prepared as monomers are polymerized, non-reacted monomers remain inside the gel, and thus, it is difficult to physically or chemically remove them after polymerizing super absorbent polymer. Thus, a method of increasing the amount of polymerization initiators when polymerizing monomers to polymerize all the monomers as possible is conventionally used. However, this method has problems in that as reaction sites increase, the strength of the polymer network of super absorbent polymer decreases, thus decreasing absorption property, and in that due to the use of excessive polymerization initiators, discoloration of super absorbent polymer appears.
Thus, the inventors, during the assiduous studies on the preparation method of super absorbent polymer capable of reducing the content of non-reacted monomers in the super absorbent polymer, confirmed that the preparation method as explained below can fulfill the above requirement, and completed the invention.